Parallel-stripe type semiconductor device

ABSTRACT

A semiconductor device has a drift region in which a drift current flows if it is in the ON mode and which is depleted if it is in the OFF mode. The drift region is formed as a structure having a plurality of first conductive type divided drift regions and a plurality of second conductive type compartment regions in which each of the compartment regions is positioned among the adjacent drift regions in parallel to make p-n junctions, respectively.

This application is a Divisional of U.S. patent application Ser. No.08/786,473, filed Jan. 21, 1997 now U.S. Pat. No, 6,097,063, which isincorporated herein by reference.

BACKGROUD OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device with a largecurrent capacity and a high breakdown voltage, such as a metal-oxidesemiconductor field-effect transistor (MOSFET), an IGBT, a bipolartransistor, and semiconductor diode, and also the present inventionrelates to a method for fabricating the semiconductor device mentionedabove.

2. Description of the Prior Art

In general, structures of the semiconductor elements can be roughlygrouped into two types: a lateral structure having an electric contactportion on one side thereof; and a vertical structure having electriccontact portions on both sides thereof.

One example of a semiconductor element with the lateral structure is aSOI (silicon on insulator) -MOSFET (metal oxide silicon field effecttransistor) shown in FIG. 1A and FIG. 1B, in which FIG. 1A is a planview of the semiconductor element, while FIG. 1B is a cross-sectionalview of the semiconductor along a line 1B-1B′ in FIG. 1A.

A structure of the SOI-MOSFET is based on an offset gate structure of ann-channel MOSFET, in which a p-type channel diffusion layer 7, ann⁻-type low doped drain region (drain/drift region) 90, and an n⁻-typedrain region 9 are formed on a semiconductor base plate 5 via aninsulation membrane 6 in that order. In addition, an n⁺-type sourceregion 8 is formed on a part of a surface of the p-type channeldiffusion layer 7, and also an insulation layer 12 is formed on a regionextending from an end portion of the region 8 to an end of the n⁺-typedrain region 9. Particularly, in close proximity to the above n⁺-typesource region 8, the insulating film 12 has a remainder portion 10 whichis thinner than the rest of the insulating film and is positioned on theboundary of the p-type channel diffusion layer 7 and the n⁻-type lowdoped drain region 90. A gate electrode 11 is formed so as to cover theinsulation film 12 from its thin portion 10 to its thick portion.

The low doped drain region 90 can be operated as a drift region fortransferring carriers due to the effect of an electrical field if theMOSFET is in the ON mode. If the low doped drain region 90 is in the OFFmode, on the other hand, it becomes a depletion region to reduce anelectric field strength applied thereon, resulting in an increase inbreakdown voltage. It is possible to reduce the drift resistance of thelow doped drain region 90 by increasing the concentration of impuritiesin the low doped drain region 90 and by decreasing electricity flowingthrough the region 90. As a result, a substantial ON resistance(drain-source resistance) of the MOSFET can be lowered. In this case,however, it will be difficult to extend the boundaries of the depletionlayer between the drain and the channel to be developed from a P-Njunction between the p-type channel diffusion layer 7 and the n-type lowdoped drain region 90, so that the maximum permissible (critical)electric field strength of silicon is reached at an earlier time,resulting in a reduction in the breakdown voltage (drain-sourcevoltage). Hence, a trade-off relationship exists between the ONresistance (current capacity) and the breakdown voltage. Similarly, ithas been known that the semiconductors such as IGBT, bipolartransistors, and semiconductor diodes make the above trade-offrelationship.

Referring now to FIGS. 2A and 2B, another example of the conventionalMOSFETs having a lateral structure will be described in detail, in whichFIG. 2A is a cross-sectional view of a p-channel MOSFET and FIG. 2B is across-sectional view of a double diffusion type n-channel MOSFET.

The p-channel MOSFET shown in FIG. 2A comprises a n-type channeldiffusion layer 3 formed on a p-type semiconductor layer 4, a gateelectrode 11 with a field plate formed on the diffusion layer 3 via agate insulation film 10, a p⁺-type source region 18 formed on a part ofthe diffusion layer 3 in close proximity to one end of the gateelectrode 11, a p-type low doped drain region (drain/drift region) 14formed as a well having an end immediately underneath the other end ofthe gate electrode 11, a n⁺-type contact region 71 adjacent to thep-type source region 18, a thick insulation film 12 formed on the p-typelow doped drain region 14. In this structure, therefore, its ONresistance and breakdown voltage can be defined as the trade-offrelationship based on the amount of electricity flowing through thewell-shaped p-type low doped drain region 14 and the concentration ofits impurities.

The double diffusion type n-channel MOSFET shown in FIG. 2B comprises an-type low doped drain layer (drain/drift layer) 22 formed on a p-typesemiconductor layer 4, a gate electrode 11 with a field plate formed onthe low doped drain layer 22 via a gate insulation film 10, a p-typechannel diffusion region 17 formed on a part of the low doped drainlayer 22 in close proximity to one end of the gate electrode 11, n⁻-typesource region 8 formed as a well in the p-type channel diffusion region17, an n⁺-type drain region 9 formed as a well positioned at a distancefrom the n⁺-type source region 8 and the gate electrode 11, awell-shaped p-type top layer 24 formed on a surface layer between thegate electrode 11 and the n⁺-type drain region 9, a p⁺-type contactregion 72 adjacent to the n⁺-type source region 8, and a thickinsulation film 12 formed on the p-type top layer 24. In this structure,therefore, its ON resistance and breakdown voltage can be defined as thetrade-off relationship based on the amount of electricity flowingthrough the well-shaped n-type low doped drain region 22 and theconcentration of its impurities.

In the structure of FIG. 2B, however, the n-type low doped drain layer22 is sandwiched between the p-type semiconductor layer 4 and the p-typetop layer 24, so that it can be provided as the structure with a highbreakdown voltage if the MOSFET is in the OFF mode because the low dopeddrain layer 22 is depleted at an earlier time by widening a depletionlayer not only from the P-N junctions ja with the p-type channeldiffusion region 17 but also from p-n junctions jb of upper and lowersides of the n-type low doped drain layer 22 while the concentration ofimpurities in the low doped drain layer 22 can be increased.

FIG. 3 shows a trench gate type n-channel MOSFET as an example of thevertical semiconductor element. The n-channel MOSFET comprises a n-typelow doped drain layer 39 formed on a n⁺-type drain layer 29 electricallycontacted with a back electrode (not shown), a trench gate electrode 21imbedded in a trench formed on a surface of the low doped drain layer 39via a gate insulation film 10, a p-type channel diffusion layer 27formed on a surface of the low doped drain layer 39 at a relativelyshallow depth compared with that of the trench gate electrode 21, an⁺-type source region 18 formed along an upper edge of the trench gateelectrode 21, and a thick insulation film 12 as a sheathing of the gateelectrode 21. By the way, it is possible to make a n-type IGBT structureusing a double structure made of a n⁻-type upper layer and a p⁺-typeunder layer instead of the single layered n⁺-drain layer 29. In Thiskind of the vertical structure, therefore, the low doped drain layer 39acts as a drift region for drift current flowing in the verticaldirection if the MOSFET is in the ON mode, while it is depleted toincrease its breakdown voltage if the MOSFET is in the OFF mode. In thiscase, its ON resistance and breakdown voltage can be defined as thetrade-off relationship on the basis of a thickness of the low dopeddrain layer 39 and he concentration of its impurities.

FIG. 4 is a graph that shows the relationship between an ideal breakdownvoltage and an ideal ON resistance of the silicon n-channel MOSFET. Inthis figure, it is based on the hypothesis that the breakdown voltagecannot be lowered by changing its form and that the ideal ON resistanceis small enough to ignore electrical resistance of the regions exceptthe low doped drain region.

In FIG. 4, the line A represents the relationship between an idealbreakdown voltage and an ideal ON resistance of the vertical structuretype n-channel MOSFET of FIG. 3; the line B represents the relationshipbetween an ideal breakdown voltage and an ideal ON resistance of an-channel type MOSFET like the MOSFET of FIG. 2A where the p-channeltype is replaced by an n-channel type; the line D represents therelationship between an ideal breakdown voltage and an ideal ONresistance of the vertically structured double diffused n-channel MOSFETof FIG. 2B; and the line C represents the relationship between an idealbreakdown voltage and an ideal ON resistance o the n-channel MOSFET ofFIG. 11.

The vertical n-channel MOSFET is constructed so as to match thedirection of the drift current flowing in the ON mode and the directionof expanding the depletion layer caused by a reverse bias in the OFFmode. For the low doped drain layer 39 of FIG. 3, an approximation ofthe breakdown voltage BV at the time of the OFF mode can be obtained bythe following equation:

BV=E_(C) ²ε₀ε_(si)α(2-α)/2qN_(D)  (1)

wherein

E_(C) is Ec (N_(D)) which is the maximum electric field strength ofsilicon at the impurity concentration of N_(D);

ε_(o) is a dielectric constant in a vacuum;

ε_(si) is a relative dielectric constant of silicon;

q is a unit charge;

N_(D) is the impurity concentration of the low doped region; and

αis a factor (0<α<1).

In addition, the ideal ON resistance per unit area in the ON mode can beapproximately obtained by the following equation:

R=αW/μqN_(C)

wherein

 μis μ(N_(D)) which is electron mobility as the impurity concentrationof N_(D); and

W is equal to E_(C) ε₀ ε_(si)/q N_(D).

Therefore, R can be represented by the following formula:

R=E_(C)ε₀ε_(si)α/μq²N_(D) ².  (2)

Thus, if qND is removed from the formula (2) using the formula (1) andif an optimum value such as ⅔ is chosen for α, the following formularesults:

R=BV²(27/8Ec³ε₀ε_(si)μ)  (3)

In this formula, the ON resistance R seems to be proportional to thesquare of the breakdown voltage BV. In this case, however, the line A ofFIG. 4 is roughly proportional to BV raised to the 2.4th to 2.6th power.

In the case of the n-channel type MOSFET structure where the channeltype of the MOSFET of FIG. 2A is replaced, the drift current flows inthe lateral direction if it is in the ON mode, while the depletion layerspreads upward (in the vertical direction) from the bottom of the wellsubstantially faster than spreading from one end of the well in thelateral direction. For obtaining a higher breakdown voltage in thedepletion layer spreading out in the vertical direction, it should bedepleted from a boundary surface of the p-n junction (i.e., the bottomof the well) between the low doped drain region 14 and the channeldiffusion layer 3 to a surface of the low doped drain region 14 (i.e.,the surface of the well), so that the maximum value of the net doping ant in the low doped drain region 14 can be restricted by the followingformula:

S_(D)=E_(c)ε₀ε_(si)/q  (4)

If the length of the low doped drain region 14 is defined as L, an idealbreakdown voltage V is represented by the formula:

BV=Ec Lβ  (5)

wherein

β is a factor (0<β<1)

In addition, the ideal ON resistance R per unit area in the ON mode canbe approximately obtained by the following equation:

R=L²/μqS_(D)  (6)

wherein

μ is μ(S_(D)) which is electron mobility at the maximum impurityconcentration of S_(D). Thus, if L is removed from the formula (6) bysubstituting the formulae (4) and (5), the following formula results:

R=BV²/β²E_(C) ³ε₀ε_(si)μ  (7)

In the case of the vertically structured double diffusion type n-channelMOSFET shown in FIG. 2B, it is constructed by forming a p-type top layer24 on the MOSFET structure of FIG. 2A. Therefore, the depletion layerspreads in the vertical direction, and thus the low doped drain layer 22can be depleted quickly. As shown in the following formula (8), the netdoping amount SD in the low doped region 22 can be increased twice asmuch as that of FIG. 2A.

 S_(D)=2E_(C)ε₀ε_(si)/q  (8)

In this case, the relationship between an ideal ON resistance R and anideal breakdown voltage of the above structure is represented by theformula:

R=BV²/2β²E_(C) ³ε₀ε_(si)μ  (9

As is evident from a comparison between the above formula (7) and theabove formula (9), the trade-off relationship (line B in FIG. 4) betweenthe ON resistance and the breakdown voltage of the vertically structuredn-channel type MOSFET of FIG. 2B is slightly improved over the trade-offrelationship (line C in FIG. 4) between the ON resistance and thebreakdown voltage of the n-channel type MOSFET where the channel type ofthe MOSFET of FIG. 2A is replaced. In this case, however, theimprovement only permits the doping concentration twice as much asbefore and it does not provide flexibility in a design criterion of thecurrent capacity and the breakdown voltage of the semiconductor.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide asemiconductor device which relaxes the relationship between the ONresistance and the breakdown voltage to enable an increase in thecurrent capacity by a reduction in the ON resistance under the highbreakdown voltage.

It is another object of the present invention to provide a manufacturingmethod for rolling out semiconductor devices.

In a first aspect of present invention, there is provided asemiconductor device having a drift region where a drift current flowsif it is in an ON mode and which is depleted if it is in an OFF mode,wherein

the drift region is formed as a structure having a plurality of firstconductive type divided drift path regions which are connected togetherin parallel to form a group of parallel drift paths and a plurality ofsecond conductive type side regions, in which each of the secondconductive type side regions is positioned between adjacent the firstconductive type divided drift path regions to form p-n junctions.

Here, the semiconductor device may further comprise:

at least one additional second side region which is connected to anouter side of a first conductive type divided drift path regionpositioned at an outer side of the group of parallel drift paths.

In a second aspect of present invention, there is provided asemiconductor device having a drift region where a drift current flowsif it is in an ON mode and which is depleted if it is in an OFF mode,and the drift current flows in a lateral direction and the drift regionis formed on a semiconductor or an insulation film on the semiconductor,wherein

the drift region is formed as a parallel stripe structure in which aplurality of stripe-shaped first conductive divided drift path regionsand a plurality of stripe-shaped second conductive type compartmentregions are alternatively arranged on a plane one by one in parallel.

In a third aspect of present invention, there is provided asemiconductor device having a drift region where a drift current flowsif it is in an ON mode and which is depleted if it is in an OFF mode,and the drift current flows in a lateral direction and the drift regionis formed on a semiconductor or an insulation film on the semiconductor,wherein

the drift region is formed as a parallel stacked structure in which aplurality of layer-shaped first conductive divided drift path regionsand a plurality of layer-shaped second conductive type compartmentregions are alternatively stacked on a plane one by one in parallel.

In a fourth aspect of present invention, there is provided asemiconductor device having a drift region where a drift current flowsif it is in an ON mode and which is depleted if it is in an OFF mode,and the drift current flows in a lateral direction and the drift regionis formed on a second conductive type semiconductor, wherein

the drift region comprises:

a first conductive type divided drift region formed on the secondconductive type semiconductor layer;

a well-shaped second conductive type compartment region formed on thefirst conductive type divided drift path region; and

a secondary first conductive type divided drift path region formed on asurface layer of the well-shaped second conductive type compartmentregion and connected to the first conductive type divided drift pathregion in parallel.

In a fifth aspect of present invention, there is provided asemiconductor device having a drift region where a drift current flowsif it is in an ON mode and which is depleted if it is in an OFF mode,and the drift current flows in a vertical direction and the drift regionis formed on a semiconductor, wherein

the drift region comprises a plurality of first conductive type divideddrift regions in which each of them has a layer structure along thevertical direction and a plurality of first conductive type compartmentregions in which each of them has a layer structure along the verticaldirection, and

the plurality of first conductive type divided drift regions and theplurality of first conductive type compartment regions are stacked oneby one in parallel in a direction perpendicular to the verticaldirection to form a laterally stacked parallel structure.

In a sixth aspect of present invention, there is provided a method ofmanufacturing a semiconductor device having a drift region where a driftcurrent flows if it is in an ON mode and which is depleted if it is inan OFF mode, and the drift current flows in a lateral direction and thedrift region is formed on a second conductive type semiconductor, wherethe drift region has: a first conductive type divided drift region armedon the second conductive type semiconductor layer; a well-shaped secondconductive type compartment region formed on the first conductive typedivided drift path region; and a secondary first conductive type divideddrift oath region formed on a surface layer of the well-shaped secondconductive type compartment region and connected to the first conductivetype divided drift path region in parallel, comprising steps of:

forming a first conductive type divided drift path region on a secondconductive type semiconductor layer made of silicon by a thermaldiffusion after performing a phosphorus ion-implantation;

forming an well-shaped second conductive type compartment region on thefirst conductive type divided drift region by a thermal diffusion afterperforming a selective boron ion-implantation; and

thermally oxidizing a structure obtained by the selective boronion-implantation to form a secondary first conductive type divided driftpath region on a surface thereof through use of a concentration ofphosphorus ions which are unevenly distributed on a surface of thesilicon and a dilution of boron ions which are unevenly distributed intoan oxidized film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view showing an example of the conventional SOI-MOSFETwhich is vertically structured;

FIG. 1B is a cross sectional view taken along line 1B—1B in FIG. 1A;

FIG. 2A is a cross sectional view showing another example of theconventional MOSFET which is vertically structured;

FIG. 2B is a cross sectional view showing an example of the conventionaln-channel MOSFET in the type of double diffusion;

FIG. 3 is a cross sectional view showing an example of the conventionaln-channel MOSFET in the type of trench gate;

FIG. 4 is a graph that illustrates the relationship between an idealbreakdown voltage and an ideal ON resistance of each silicon n-channelMOSFET;

FIG. 5A is a perspective illustration of the first example of a driftregion structure of a semiconductor device in accordance with thepresent invention;

FIG. 5B is a perspective illustration of the second example of the driftregion structure of the semiconductor device in accordance with thepresent invention;

FIG. 5C is a perspective illustration of the third example of the driftregion structure of the semiconductor device in accordance with thepresent invention;

FIG. 6A is a plan view showing a SOI-MOSFET in the type of verticallystructured as the first preferred embodiment of the semiconductor devicein accordance with the present invention;

FIG. 6B is a cross sectional view taken along line 6B—6B in FIG. 6A;

FIG. 6C is a cross sectional view taken along line 6C—6C in FIG. 6A;

FIG. 7A is a plan view showing a SOI-MOSFET in the type of doublediffusion as the second preferred embodiment of the semiconductor devicein accordance with the present invention;

FIG. 7B is a cross sectional view taken along line 7B—7B in FIG. 7A;

FIG. 7C is a cross sectional view taken along line 7C—7C in FIG. 7A;

FIG. 8A is a plan view showing a SOI-MOSFET in the type of verticallystructured as the third preferred embodiment of the semiconductor devicein accordance with the present invention;

FIG. 8B is a cross sectional view taken along line 8B—8B in FIG. 8A;

FIG. 8C is a cross sectional view taken along line 8C—8C in FIG. 8A;

FIG. 9A is a plan view showing a MOSFET in the type of verticallystructured as the fourth preferred embodiment of the semiconductordevice in accordance with the present invention;

FIG. 9B is a cross sectional view taken along line 9B—9B in FIG. 9A;

FIG. 9C is a cross sectional view taken along line 9C—9C in FIG. 9A;

FIG. 10 is a cross sectional view showing a p-channel MOSFET in the typeof vertically structured as the fifth preferred embodiment of thesemiconductor device in accordance with the present invention;

FIG. 11 is a cross sectional view showing an n-channel MOSFET in thetype of vertically structured as the sixth preferred embodiment of thesemiconductor device in accordance with the present invention;

FIG. 12A is a plan view showing a trench gate n-channel MOSFET in thetype of vertically structured as the seventh preferred embodiment of thesemiconductor device in accordance with the present invention;

FIG. 12B is a cross sectional view taken along line 12B—12B in FIG. 12A;

FIG. 12C is a cross sectional view taken along line 12C—12C In FIG. 12A;

FIG. 12D is a cross sectional view taken along line 12D—12D in FIG. 12B;

FIG. 12E is a cross sectional view taken along line 12E—12E in FIG. 12A;and

FIG. 12F is a cross sectional view taken along line 12F—12F in FIG. 12A.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to the present invention, a semiconductor device has a driftregion where a drift current flows in the ON mode and which is depletedin the OFF mode. The drift region is formed as a structure having aplurality of divided substructures (i.e., divided regions) in parallel,such as a layered structure, a fiber structure, or a honeycombstructure, as shown in FIGS. 5A to 5C. In addition, the drift regioncomprises a plurality of the first conductive type divided drift pathregions 1 and a plurality of the second conductive type compartmentregions 2 in which each of the regions 2 is positioned among theadjacent regions 1 to make p-n junctions.

In FIG. 5A, for example, the drift region comprises a group of paralleldrift path substructures (i.e., complex structure) 100 consisting of: atleast two 1st type (e.g., n-type) divided drift path regions 1 whereeach of them is in the shape of a plate connected to another plate inparallel at least at its one end; and at least one 2nd conductive type(e.g., p-type) compartment region 2 being sandwiched between the divideddrift path regions 1,1 so as to make p-n junctions. In this figure, aplurality of 2nd type compartment regions 2 is shown. These 2ndcompartment regions are connected in parallel with each other by theirend portions.

The drift region shown in FIG. 5B is in the type of a multiple fiberstructure. It comprises a plurality of the first conductive type (e.g.,n-type) drift path regions 1 and a plurality of the second conductivetype (e.g., p-type) compartment. Each region 1 or 2 is in the shave of afiber. In a bundle of fibers (i.e., drift regions), the regions 1 and 2are arranged so as to form a check pattern thereof in cross-section.

Furthermore, for the drift region shown in FIG. 5C, the is: conductivetype (e.g., n-type) divided drift path region 1 has connecting portionsla in the four corners of the cross-section.

Additional 2nd conductive side regions 2 a may be provided on theoutermost surfaces (i.e., top and bottom surfaces) of the complexstructure 100 as shown in FIG. 5A or in four corners of the complexstructure 190 as shown in FIG. 5B.

If the semiconductor device is in the ON mode, a drift current flowsthrough a plurality of the divided drift path regions 1,1 arranged inparallel. If the semiconductor device is in the OFF mode, on the otherhand, a depletion layer spreads out from each p-n junction between thefirst conductive type divided drift region 1 and the second conductivetype compartment region 2 into the region 1 to reduce the density ofcharge carriers therein. In this case, the depletion can be acceleratedby laterally extending the outermost ends of the depletion region (i.e.,the region in which there are no charge carriers) from both longitudinalsides of the second conductive compartment region, wherein the secondconductive compartment region 2 is simultaneously depleted.Consequently, a breakdown voltage of the semiconductor device becomeshigh, so that the impurity concentration of the n-type drift path region1 can be increased to reduce the ON resistance. Especially in thepresent invention, as described above, the depletion region can bewidened from both longitudinal sides of the second conductive typecompartment region 2 into the adjacent regions 1,1, respectively.Elongating ends of the depletion region act effectively on therespective divided drift path regions 1,1, so that the total width ofthe second conductive type compartment region 2 to be required forforming the depletion layer may be reduced, while the cross sectionalarea of the first conductive divided drift path region 1 may beincreased by about the same extent, resulting in the drop in the ONresistance compared with the conventional device. Accordingly, it ispreferable that the second conductive type compartment region 2 isprepared so as to have a comparatively small width thereof as much aspossible. It is also preferable that the impurity concentration of thesecond conductive type compartment region 2 be low as much as possible.In addition, the trade-off relationship between the ON resistance andthe breakdown voltage can be eased if the number of the first conductivetype divided drift path regions 1 per unit area (i.e., the number ofdivided regions per unit area) is increased.

In the present invention, an equation that represents the trade-offrelationship between an ideal ON resistance r and a breakdown voltage BVfor each of the first conductive type divided drift path regions 1corresponds to the following formula (10) obtained by modifying theformula (9) on the assumption that the width of the second conductivecompartment region 2 is infinitely small, in which the ON resistance ris N times higher than the ideal ON resistance R.

r=NR=BV²/2β²E_(C) ³ε_(c)ε_(Si)μ  (10)

The relationship between an ideal ON resistance R and an ideal breakdownvoltage BV of the complex structure of drift path substructures arrangedin parallel can be represented by the formula:

R=BV²/2Nβ²E_(C) ³ε_(o)ε_(Si)μ  (11)

Therefore, the possibility of manufacturing a semiconductor devicehaving a considerably low ON resistance can be increased in a directproportional relationship with the number of the divided drift regions.

In much the same fashion as a lateral type semiconductor device formedon a silicon-on-insulator (SOI) or a semiconductor layer, a lateral typesemiconductor device having a drift region formed on a semiconductorlayer or on an insulation film on the semiconductor layer, in which adrift current flows in the drift region in the lateral direction if itis in the ON mode and mobile charges are depleted if it is in the OFFmode. The drift region can be fabricated as a stripe structure byalternately arranging the respective first conductive type divided driftpath regions in the shape of a stripe and the respective secondconductive type compartment regions in the shape of a stripe on a plane.The stripe shaped p-n junction repeated structure on the plane may beformed by performing a photolithography one time, resulting in a simplemanufacturing process and a low production cost for the semiconductordevice.

Another structure of the drift region to be formed in the lateral typesemiconductor device may be a superposed parallel structure byalternatively laminating the respective first conductive type divideddrift path region in the shape of a flat layer and the respective secondconductive type compartment region in the shape of a flat layer. Thethickness of each layer can be precisely decreased as much as possibleusing a metal organic chemical vapor deposition (MOCVD) or a molecularbeam epitaxy (MBE), so that the trade-off relationship between the ONresistance and the breakdown voltage can be substantially eased.

By the way, it may be possible to prepare the drift region as thesuperposed structure with the striped parallel structure.

If N=2 in the above formula (10) or (11), the complex structure ofparallel drift paths is made of two stripe-shaped divided drift pathregions. Thus the most simple drift region of the lateral typesemiconductor device comprises a first conductive type divided driftpath region formed on a second conductive type semiconductor layer, awell-shaped second conductive type compartment region formed on thefirst conductive type divided drift path region, another firstconductive type divided drift path region formed on a surface layer ofthe second conductive type compartment region and connection to thefirst conductive type divided drift path region. The ON resistance ofthe semiconductor device can be reduced by connecting the another firstconductive type divided drift path region with the first conductive typedivided drift path region in parallel.

A method for fabricating the above simple lateral type semiconductorcomprises the seeps of: forming a first n-type divided drift oath regionon a p-type semiconductor layer on a silicon layer by a thermaldiffusion after performing a phosphorus ion-implantation; forming awell-shaped p-type compartment region on the first n-type divided driftpath region by a thermal diffusion after performing a selective boronion-implantation; thermally oxidizing the obtained layer structure toform a second n-type divided drift path region on a surface thereofthrough the use of concentrated phosphorus ions which are unevenlydistributed on a surface of the silicon and of diluted boron ions whichare unevenly distributed into the oxidized film.

There is no reverse conductive type layer adjacent to the top layer ofthe second n-type divided drift path region. Hence, it is enough toprovide a thin layer to readily deplete the second n-type divided driftpath region. The fabrication method of the present invention does notrequire the step of doping impurities. Further, the second n-typedivided drift region is provided only by the step of the thermaloxidation, which contributes to reduced cost and a reduced number ofsteps for providing a way for the practical mass production ofsemiconductor devices.

Furthermore, another semiconductor device according to the presentinvention has a drift region formed on a semiconductor layer, in whichthe drift region feeds a drift current in the vertical direction if thedevice is in the ON mode and the drift region is depleted if the deviceis in the OFF mode. Exemplary devices include vertical typesemiconductor devices including a semiconductor using a trench gate orthe like and IGBT. The drift region comprises a plurality of firstconductive type divided drift path regions and a plurality of secondconductive compartment regions, in which each region is in the shape ofa layer in the vertical direction. The respective first conductive typedivided drift oath regions and the respective second conductive typecompartment regions are alternatively laminated in parallel in thelateral direction. In the process for fabricating this structure, anetching step can be required for forming a deep groove. In this case,however, it is also possible to substantially ease the trade-offrelationship between the ON resistance and the breakdown voltage of thevertical type semiconductor device.

Embodiment 1

Referring now to FIGS. 6A to 6C, a silicon-on-insulator (SOI) metaloxide semiconductor field-effect transistor (MOSFET) in the type of alateral structure (hereinafter, referred as a lateral SOI MOSFET) willbe described in detail as a first preferred embodiment of the presentinvention. In these figures, FIG. 6A is a plan view of the lateralSOI-MOSFET, FIG. 6B is a cross sectional view along a line 6B—6B in FIG.6A, and FIG. 6C is a cross sectional view along a line 6C—6C in FIG. 6A.

The lateral SOI-MOSFET of the present embodiment has the same structureas the offset gate structure of the n-channel MOSFET shown in FIGS. 1Aand 1B except in the structure of drain/drift region.

A structure of the lateral SOI-MOSFET comprises a p-type channeldiffusion layer 7 formed on an insulation film 6 on a semiconductorsubstrate 5, a gate electrode 11 with a field plate formed on thechannel diffusion region 7 via an insulation film 10, an n⁺-type sourceregion 8 formed on a portion of the channel diffusion layer 7 where theportion is on the side of one end of the gate electrode 11, an n⁺-typedrain region 9 formed on a position at a predetermined distance from theother end of the gate electrode 11, a drain/drift region 190 which isextended between the diffusion region 7 and the drain region 9, and athick insulation film 12 formed on the drain/drift region 190.

The drain/drift region 190 of the present embodiment consists of aplurality of divided regions in the shape of a stripe: n-type drift pathregions 1 and p-type compartment regions 2, which are alternatelyarranged in parallel on a plane to form a parallel stripe structure. Oneend of each n-type drift path region 1 is connected to the p-typechannel 7 diffusion region 7 to form a p-n junction, while the other endthereof is connected to the n⁺-type drain region 9. Thus the n-type pathregions 1 arranged in parallel form a drift path group 100 branched offfrom the n^(/+)-type drain region 9. In addition, as shown in thefigure, there is a p-type semiconductor region 2 a adjacent to alongitudinal side of the drift region 1 positioned at each side of thedrift path group, and also each of the drift regions 1 is sandwichedbetween p-type semiconductor regions 2 (2 a). Furthermore, one end ofeach p-type semiconductor is connected to the p-type channel diffusionregion 7, while the other end thereof is connected to the n⁺-type drainregion 9 to form a p-n junction. Therefore, the respective p-typecompartment regions 2 are branched off from the p-type channel diffusionregion 7 and form parallel connection with the respective n⁺-type drainregions 9.

If the lateral SOI-MOSFET is in the ON mode, carriers (electrons) flowfrom the n⁺-type source region 8 into a plurality of the n-type driftpath regions 1 through a channel inversion layer directly below the gateinsulation film 10 to cause a drift current by an electric fieldgenerated by voltage placed between the drain and the source. If it isin the OFF mode, on the other hand, the channel inversion layer 13directly below the gate insulation film 10 disappears. Further, adepletion layer is widened from the p-n junction ja between the n-typedrift path region 1 and the p-type channel diffusion region 7, and adepletion layer is widened from the p-n junction jb between the n-typedrift path region 1 and the p-type compartment region 2 into the n-typedrift path region 1, resulting in a depletion of the n-type drift pathregion 1. In this case, one end of the depletion layer is widened fromthe p-n junction ja along a path length in the n-type drift path region1 and the other end thereof is widened from the p-n junction jb along apath width in the n-type drift path region 1. That is, the depletionlayer is widened from both sides to accelerate the depletion. Therefore,the electric field strength is weakened and the breakdown voltagebecomes high, so that the concentration of impurities in the respectiven-type drift path regions 1 may be increased. In this embodiment,particularly, the ends of the depletion are extended from both of thelongitudinal sides of the p-type compartment region 2 into the adjacentn-type drift path regions 1, 1, respectively, and thus the total widthof the p-type compartment regions 2 can be reduced in half, while thecross sectional area of the n-type drift path region 1 can be increased.This results in the drop in the ON-resistance in comparison with that ofthe conventional device. In addition, the trade-off relationship betweenthe ON resistance and the breakdown voltage is extensively weakened asthe number N of the n-type drift path region 1 per unit area isincreased. It is preferable that the width of the p-type compartmentregions 2 is as small as possible.

For the sake of clarity, the ON resistance R of the lateral SOI-MOSFETof the present embodiment will be compared with that of the conventionalone under the following condition: the ideal breakdown voltage BV=100 V;the concentration of impurities in the first n-drift path region 1N_(D)=3×10¹⁵ (cm⁻³); the maximum electric field strength of siliconE_(C)=3×10⁵ (V/cm); the mobility of electron μ=1,000 (cm²/V sec); thedielectric constant of vacuum ε₀=8.8×10⁻¹² (C/Vm); the relativedielectric constant of silicon ε_(si)=12; and the unit chargeq=1.6×10⁻¹⁹ (C).

In the case of the lower doping drain region 90 of the conventionaldevice shown in FIG. 1A, the ideal ON resistance R is 9.1 (m·ohm·cm²)using the equations described above if the region 90 has the length of6.6 μm and the thickness of 1 μm. In the case of the present embodiment,on the other hand, the ideal ON resistance R is dramatically droppedwhen the width W of each of the n-type drift path region 1 and thep-type compartment region 2 is less than 1 μm. That is, R=7.9(m·ohm·cm²) when W=10 μm; R=0.8 (m·ohm ·cm²) when W=1 μm; and R=0.08(m·ohm·cm²) when W=0.1 μm if the length thereof is 5 μm, and βis ⅔. Ifthe width of the p-type compartment region 2 is slightly larger thanthat of the n-type drift path region 1, a noticeable improvement in theideal ON resistance R can be further obtained. For the mass-productionof semiconductor devices, by the way, it is difficult to obtain thewidth of each region 1 or 2 less than 0.5 μm by means ofphotolithography and ion-implantation at the present time. In the nearfuture, however, further reduction in the ON resistance of the lateralSOI-MOSFET of the present embodiment will be achieved by lessening thewidth of each region 1 or 2 less than 0.5 μm as micro-machiningtechnology progresses.

The structure of the drift region to be applied in the presentembodiment is repeated p-n junctions of stripes on a plane, so that thestructure can be processed by a single step of photolithography for asimplification of the manufacturing process to provide chips at thelowest cost.

Embodiment 2

Referring now to FIGS. 7A to 7C, a double diffused type n-channel MOSFET(hereinafter, also referred as a double diffused MOSFET) will bedescribed in detail as a second preferred embodiment of the presentinvention. In these Figures, FIG. 7A is a plan view showing the doublediffused MOSFET, FIG. 7B is a cross sectional view along a line 7B—7B inFIG. 7A, and FIG. 7C is a cross sectional view along a line 7C—7C inFIG. 7A.

The double diffused MOSFET of the present embodiment has the samestructure as the conventional double diffused MOSFET shown in FIGS. 2Aand 2B except in the structure of drain/drift region. As shown in theFigure, the double diffused MOSFET of the present embodiment comprises adrain/drift region 122 formed on a p-type or n⁻-type semiconductor layer4, a gate electrode 11 with a field plate formed on the drain/driftregion 122 through a gate insulation film 10, a p-type channel diffusionregion 17 in the shape of a well formed on a portion of thesemiconductor layer 4 where the portion is at one end of the gateelectrode 11, an n⁺-type source region 8 in the shape of a well formedin the p-type channel diffusion region 17, an n⁺-type drain region 9formed on a position at a predetermined distance from the other end ofthe gate electrode 11, a drain/drift region 122 which is extendedbetween the p-type channel diffusion region 17 and the n⁺-type drainregion 9, and a thick insulation film 12 formed on the drain/driftregion 122.

The drain/drift region 122 of the present embodiment consists of aplurality of divided regions shaped as stripes, in the same way as thefirst preferred embodiment shown in FIGS. 6A to 6C: n-type drift pathregions 1 and p-type compartment regions 2, which are alternatelyarranged in parallel on a plane to form a parallel stripe structure. Oneend of each n-type drift path region 1 is connected to the p-typechannel diffusion region 17 to form a p-n junction, while the other endthereof is connected to the n⁺-type drain region 9. Thus, the n-typepath regions 1 arranged in parallel form a drift path group 100 branchedoff from the n⁺-type drain region 9. In addition, as shown in thefigure, there is a p-type semiconductor region 2 a adjacent to alongitudinal side of the drift region 1 positioned at each side of thedrift path group, and also each of the drift regions 1 is sandwichedbetween p-type semiconductor regions 2, 2 a. Furthermore, one end ofeach p-type semiconductor is connected to the p-type channel diffusionregion 17, while the other end thereof is connected to the n⁺-type drainregion 9 to form a p-n junction. Therefore, the respective p-typecompartment regions 2 are branched off from the p-type channel diffusionregion 17 and form parallel connections with the respective n⁺-typedrain regions 9.

If the double diffused MOSFET is in the OFF mode, just like the firstembodiment, the channel inversion layer 13 directly below the gateinsulation film 10 disappears. Further, a depletion layer is widenedfrom the p-n junction ja between the n-type drift path region 1 and thep-type channel diffusion region 7, and a depletion layer is widened fromthe p-n junction jb between the n-type drift path region 1 and thep-type compartment region 2 into the n-type drift path region 1,resulting in a depletion of the n-type drift path region 1. In thiscase, one end of the depletion layer is widened from the p-n junction jbalong a path length in the n-type drift path region 1 and the other endthereof is widened from the p-n junction jb along a path width in then-type drift path region 1. That is, the depletion layer is widened fromboth sides to accelerate the depletion. Therefore, the breakdown voltagebecomes high, so that the concentration of impurities in the respectiven-type drift path regions 1 may be increased to drop in the ONresistance.

For the sake of clarity, the ON resistance R of the double diffusedMOSFET of the present embodiment will be compared with that of theconventional one shown in FIG. 2B under the same conditions as that ofthe first embodiment at the ideal breakdown voltage BV=100 V. In thecase of the conventional device shown in FIG. 22, the ideal ONresistance R is about 0.5 (m·ohm·cm²) In the case of the presentembodiment, on the other hand, the ideal ON resistance R is 0.4(m·ohm·cm²), if each of the drift path region 1 and the compartmentregion 2 has a thickness of 1 mm and a width of 0.5 μm. It is possibleto drop the ON resistance extensively by further narrowing a width ofeach region 1 or 2. Alternatively, it is also possible to drop the ONresistance extensively by enlarging a resistance cross-section of thedrift path region 1 by thickening the respective drift path regions 1and the respective p-type compartment region 2. For example, the ONresistance R can be {fraction (1/10)} of the conventional one if athickness of the region 1 or 2 is 10 μm and it can be {fraction (1/100)}of the conventional one if a thickness of the region 1 or 2 is 100 μm.For the doping into such a thickened region, an impurityion-implantation with a plurality of energy levels (or successive energylevels) may be performed on the same portion of the thickened region.

Embodiment 3

FIGS. 8A to 8C show a lateral SOI-MOSFET as a third preferred embodimentof the present invention. In these figures, FIG. 8A is a plan view ofthe lateral SOI-MOSFET, FIG. 8B is a cross sectional view along a line8B—8B in FIG. 8A, and FIG. 8C is a cross sectional view along a line8C—8C in FIG. 8A.

The lateral SOI-MOSFET of the present embodiment comprises a p-typechannel diffusion layer 77 formed on a semiconductor substrate 5 throughan insulation layer 6, a trench gate electrode 111 formed on the p-typechannel diffusion layer 77 through a gate insulation film 10, aplurality of n⁺-type source regions 88 formed in the top side of thep-type channel diffusion layer 77 and adjacent to an upper edge of thetrench gate electrode 111, an n⁻-type drain region 99 formed on aposition at a predetermined distance from the gate electrode 111, adrain/drift region 290 which is extended between the drain region andthe gate electrode; and a thick insulation film 12 formed on thedrain/drift region 290.

The drain/drift region 290 of the present embodiment, as distinct fromthat of the first embodiment, is provided as a stacked layer structurein which the respective n-type drift path regions 1 and the respectivep-type compartment regions 2 are alternately stacked in parallel,repeatedly. In this case, each of these regions 1, 2 is in the shape ofa plate. As shown in the figure, an additional p-type compartment region2 a is positioned as a bottom end region of the drain/draft region 290at the side of the bottom n-type drift path region 1, and also anotheradditional p-type compartment region 2 a is positioned as a top endregion of the drain/drift region 290 at the side of the top n-type driftpath region 1. A net doping concentration of each of the regions 2 a isless than 2×10¹²/cm². One end of each of the respective n-type driftpath regions 1 is connected to the p-type channel diffusion layer 77 toform a p-n junction, while the other end thereof is connected to then⁺-type drain region 99. Thus the n-type drift path regions 1 arrangedin parallel form a drift path group 100 which is branched off from then⁻-type drain region 99. In addition, as shown in the figure, one end ofeach of the p-type compartment regions 2 is connected to the p-typechannel diffusion layer 77, while the other end thereof is connected tothe n⁺-type drain region 99 to form a p-n junction. Thus, the p-typecompartment regions 2 are branched off from the p-type channel diffusionlayer 77 and arranged in a parallel connection.

In this embodiment, furthermore, an ideal ON resistance of the lateralSOI-MOSFET can be calculated by the formula (11) described above. Inthis case, N is the number of the stacked n-type drift path regions 2.If the ideal breakdown voltage is 100V, the ideal ON resistance R is 0.5(m·ohm·cm²) for the conventional structure (N=1) but 0.05 (m·ohm·cm²)for the present structure (N=10). It means that the ON resistance R issubstantially dropped in inverse proportion to the number N of dividedregions 1.

As described above, basic technologies for fabricating the structuresshown in FIGS. 6A to 6C and FIGS. 7A to 7C are photolithography and ionimplantation. In this embodiment shown in FIGS. 8A to 8C, on the otherhand, a crystal growth technique is used because the plate-shapedregions 1, 2 should be stacked in alternate order. A total thickness ofthe whole regions 1, 2 and a period of performing the crystal growth areincreased in proportionate to the number of the regions 1, 2 to bestacked. Thus, an unequal distribution of the impurities cannot beignored because the impurities tend to diffuse in the respective thickenregions. Preferably, a thickness of each regions 1, 2 should be reducedas much as possible to perform the crystal growth at a low temperatureenough to ignore the unequal distribution. Comparing with an epitaxialgrowth heavily used in the conventional silicon-processing technologies,it is preferable to use a metal organic chemical vapor deposition(MOCVD) and a molecular beam epitaxy (MBE), which are generally appliedin the fabrication of compound semiconductors such as a gallium arsenidesemiconductor, in the present embodiment. These techniques can beprovided as micro-machining techniques which contribute to reduce the ONresistance by an effect of thinning the plate-shaped n-type drift pathregions 1 and the plate shaped p-type compartment regions 2.

In this embodiment, by the way, the difficulty of forming a channelinversion layer 13 is increased if the concentration of impurities isincreased by thinning those regions 1, 2. Consequently, it is difficultto drop the ON resistance because of the difficulty of lowering thechannel resistance. To solve this problem, it is preferable to make alow concentrated area on a part of the region where the gate insulationmembrane 10 touches one of the n-type drift regions 1 and the p-typecompartment regions 2.

Embodiment 4

Referring now to FIGS. 9A to 9C, a lateral MOSFET will be described indetail as a fourth preferred embodiment of the present invention. Inthese figures, FIG. 9A is a plan view showing the lateral MOSFET, FIG.9B is a cross sectional view along a line 9B—9B in FIG. 9A, and FIG. 9Cis a cross sectional view along a line 9C—9C in FIG. 9A.

The lateral MOSFET of the present embodiment comprises a p-type channeldiffusion layer 77 formed on a p⁻- or n⁻-type semiconductor substrate 4,a trench gate electrode 111 formed on a side wall of the p-type channeldiffusion layer 77 through a gate insulation film 10, a plurality ofn⁺-type source regions 88 formed in the top end of the p-type channeldiffusion layer 77 and adjacent to an upper edge of the trench gateelectrode 111, a n⁺-type drain region 99 formed on a position at apredetermined distance from the gate electrode 111, a drain/drift region290 which is extended between the drain region and the gate electrode;and a thick insulation film 12 formed on the drain/drift region 290.

The drain/drift region 290 of the present embodiment, as the same asthat of the third embodiment, is provided as a stacked layer structurein which the respective n-type drift path regions 1 and the respectivep-type compartment regions 2 are alternately stacked in parallel,repeatedly. In this case, each of these regions 1, 2 is in the shape ofa plate. As shown in the figure, an additional p-type compartment region2 a is positioned as a bottom end region of the drain/drift region 290at the side of the bottom n-type drift path region 1, and also anotheradditional p-type compartment region 2 a is positioned as a bottom endregion of the drain/drift region 290 at the side of the to p-n-typedrift path region 1. A net doping concentration of each of the regions 2a is less than 2×10¹²/cm². One end of each of the respective n-typedrift path regions 1 is connected to the p-type channel diffusion layer77 to form a p-n junction, while the other end thereof is connected tothe n⁺-type drain region 99. Thus the n-type drift path regions 1arranged in parallel form a drift path group 100 which is branched offfrom the n⁺-type drain region 99. In addition, as shown in the figure,one end of each of the p-type compartment regions 2 is connected to thep-type channel diffusion layer 77, while the other end thereof isconnected to the p-type drain region 99 to from a p-n junction. Thus thep-type compartment regions 2 are branched off from the p-type channeldiffusion layer 77 and arranged in a parallel connection.

In this embodiment, as in the case of the third embodiment, it ispossible to reduce the ON resistance and to increase the breakdownvoltage. The relationship between the structure of the presentembodiment and that of the third embodiment shown in FIGS. 8A to 8Ccorresponds to the relationship between the second embodiment shown inFIGS. 7A to 7C and the first embodiment shown in FIGS. 6A to 6C. Thatis, the structure of the present invention is not of SOI, so that it ispossible to fabricate the semiconductor device at a low cost.

Embodiment 5

FIG. 10 is a cross sectional view of a lateral p-channel MOSFET as afifth preferred embodiment of the present invention, corresponding tothat of FIG. 2A except for the drain/drift region.

The lateral p-channel MOSFET of the present embodiment comprises an-type channel diffusion layer 3 formed on a p⁻-type semiconductor layer4, a gate electrode 11 with a field plate formed on the n-type channeldiffusion layer 3 through a gate insulation film 10, a p⁺-type sourceregion 18 in the shape of a well formed on a portion of the n-typechannel diffusion region 3 were the portion is at one end of the gateelectrode 11, a p-type drain,/drift region 14 in the shape of a wellformed in the n-type channel diffusion region 3 where a portion of theregion 14 is directly below the other end of the gate electrode 11, an-type compartment region 2 b provided as a top side region formed on asurface of the p-type drain/drift region 14, a p⁻-type drain region 19formed on a position at a predetermined distance from the other end ofthe gate electrode 11, an n⁺-type contact region 71 adjacent to thep⁺-type source region 18, and a thick insulation film 12 formed on thep-type drain/drift region 14. In this embodiment, the number of divideddrain regions N is one (1), so that the p-type drain/drift region 14corresponds to a stripe of the drain path region 1 in the crosssectional view. A thickness of the n-type top side region 2 b on thep-type drain/drift region 14 is formed as a thin film for the purpose ofexpediting the depletion. Comparing this embodiment with the structureof FIG. 2A, the n-type top side region 2 b is provided in the presentstructure for accelerating the depletion by providing a depletion layerfrom the n-type channel diffusion layer 3 under the p-type drain/driftregion 14 and another depletion layer from the n-type top side region 2b above the p-type drain/drift region 14. The net doping concentrationof the drain/drift region 14 of the conventional structure shown in FIG.2A is approximately 1×10¹²/cm², while the structure of the presentinvention has the net doping concentration of approximately 2×10¹²/cm ²which is more than twice as much as that of the conventional one.According to the present embodiment, therefore, it is possible to reducethe ON resistance as a result of increasing the concentration ofimpurities in the drain/drift region, and it is possible to increase thebreakdown voltage.

Embodiment 6

FIG. 11 is a cross sectional view showing a double diffused n-channelMOSFET in the type of a lateral structure (hereinafter, simply referredas a double diffused MOSFET) as a sixth preferred embodiment of thepresent invention, corresponding to that of FIG. 2B except for thedrain/drift region.

The double diffused MOSFET comprises a drain/drift region 22 (i.e., afirst n-type drift path region 1) formed on a p-type semiconductor layer4 (i.e., a p-type bottom side region 2 a), a gate electrode 11 with afield plate formed on the drain/drift region 22 through a gateinsulation film 10, a p-type channel diffusion region 17 in the shape ofa well formed on a portion of the drain/drift region 22 where the region17 is positioned at one end of the gate electrode 11, an n⁺-type sourceregion 8 in the shape of a well formed in the p-type channel diffusionregion 17, a p-type top layer 24 (i.e., a p-type compartment region 2)formed on a surface layer between the gate electrode 11 and the n⁺-typedrain region 9 positioned at a predetermined distance from the gateelectrode 11, a second n-type drift path region 1 formed on a surface ofthe p-type compartment region 2, a p⁻-type contact region 72 adjacent tothe n⁺-type source reckon 8, and a thick insulation layer 12 formed onthe p-type compartment region 2.

The drain/drift region 22 as a lower layer and the n-type drift pathregion 1 as an upper layer are contacted together in parallel throughthe p-type compartment region 2. Comparing the present embodiment withthe structure of FIG. 2B, the n-type drift path region 1 is additionallyprovided on the p-type compartment region 2. As described above, it ispossible to increase the breakdown voltage as a result of widening thedepletion layers from the p-type compartment region 2 to the drain/driftregion 22 as the under layer thereof and to the drift path region 1 asthe upper layer thereof, respectively, resulting in a drop in the ONresistance. The net doping concentration of the drain/drift region 22 ofthe structure shown in FIG. 2B is approximately 2×10¹²/cm², while thestructure of the present invention has the net doping concentration(i.e., the sum of doping concentration of the under layered drain/driftregion 22 and the upper layered drift path region 1) of approximately3×10¹²/cm² which is 1.5-fold the concentration of the conventional one.According to the present embodiment, therefore, it is possible to obtainthe trade-off relationship between the ideal breakdown voltage and theideal ON resistance represented by the line D in FIG. 4. As is evidentfrom the above description, the above trade-off relationship can beeased by the present structure compared with the conventional one.

A method for fabricating the structure of each of the fifth and sixthembodiments includes the steps of: forming a n-type semiconductor layer3 (22) by implanting phosphorus ions into a p-type semiconductor andsubjecting to a heat treatment (i.e., thermal dispersion)) forming ap-type region 14 (24) on a surface of the n-type semiconductor layer 3(22) by selectively implanting boron ions and subjecting to a heattreatment (i.e., thermal dispersion), and subjecting the obtainedintermediate structure to a thermal oxidization co form a thin n-typetop side region 2 b (n-type drift path region 1) on a surface layerthrough the use of concentrated phosphorus ions which are unevenlydistributed on a surface of the silicon and diluted boron ions which areunevenly distributed into the oxidized film. In this case, there is noreverse conductive type layer adjacent to the upper layer of the n-typedrift path region 1 or the n-type top side region 2 b. Hence, it isenough to provide a thin layer to readily deplete the second n-typedrift path region. The fabrication method of the present embodiment doesnot require the step of doping impurities and it provides the n-type topside region 2 b (n-type drift path region 1) only by the step of thethermal oxidation, contributing a way for reducing the total number ofsteps and the practical mass production of semiconductor devices.

In the fifth preferred embodiment, the gate insulation film 10 and thedrain/drift region 14 are separated by the n-type top side region 2 bbecause the n-type top side region 2 b is formed on a substantiallywhole surface of the silicon surface layer using the above fabricationmethod. In this case, however, no problem occurs because the drain/driftregion 14 can be electrically conducted by a channel inversion layerformed directly underneath the gate 10 if the n-type top side region 2 bis formed as a thin film.

Embodiment 7

FIGS. 12A to FIGS. 12F show a trench gate n-channel MOSFET in the typeof a vertical structure (hereinafter, referred as a vertical MOSFET) asa seventh embodiment of the present invention. In these figures, FIG.12A is a plan view showing the vertical MOSFET; FIG. 12B is a crosssectional view along a line 12B—12B in FIG. 12A; FIG. 12C is a crosssectional view along a line 12C—12C in FIG. 12A; FIG. 12D is a crosssectional view along a line 12D—12D in FIG. 12A; FIG. 12E is a crosssectional view along a line 12E—12E in FIG. 12A; and FIG. 12F is a crosssectional view along a line 12F—12F in FIG. 12A.

The vertical MOSFET comprises an n⁺-type drain layer 29 electricallycontacted to a back electrode (not shown), a drain/drift region 139formed on the n⁻-type drain layer 29, a trench gate electrode 21imbedded in a trench formed on a surface of the drain/drift region 139via a gate insulation film 10, a p-type channel diffusion layer 27formed on a surface of the drain/drift region 139 at a relativelyshallow depth compared with that of the trench gate electrode 21, an⁺-type source region 29 formed along an upper edge of the trench gateelectrode 21, and a thick insulation film 12 as a sheathing of the gateelectrode 21. By the way, it is possible to make a n-type IGBT structureusing a p-type layer or a double layered structure made of an n⁺-typeupper layer and a p⁺-type under layer instead of the single layeredn⁺-type drain layer 29.

According to the present embodiment, as shown in FIGS. 12D to 12F, thedrain/drift region 139 comprises a plurality of plate-shaped dividedregions in the vertical direction, in which n-type drift path regions 1and p-type compartment regions 2, which are alternately arranged inparallel in the vertical direction to form a parallel stripe structure.An upper end of each the n-type drift path regions 1 is connected to thep-type channel diffusion layer 27 to form a p-n junction, while a lowerend thereof is connected to the n⁺-type drain layer 29. Thus the n-typedrift path regions 1 are arranged in parallel to form a drift path group100 branched off from the n⁺-type drain layer 29. In addition, not shownin the figure, there is a p-type semiconductor side region adjacent to alongitudinal side of a n-type drift path region 1 positioned at eachside of the drift path group 100, and also each of the n-type drift pathregions 1 is sandwiched between p-type semiconductor side regions orp-type compartment regions. Furthermore, the upper end of each of thep-type compartment regions 2 is connected to the p-type channeldiffusion layer 27, while the lower end thereof is connected to then⁺-type drain layer 29 to form a p-n junction. Therefore, the respectivep-type compartment regions 2 are branched off from the p-type channeldiffusion region 27 and form a parallel connection with the respectiven⁺-type drain regions 29.

If the vertical MOSFET is in the OFF mode, the channel inversion layer13 directly below the gate insulation film 10 disappears. Due to thepotential between the drain and the source, in addition, depletionlayers are widened from the p-n junction ja between the n-type driftpath region 1 and the p-type channel diffusion region 27 and the p-njunction jb between the n-type drift path region 1 and the p-typecompartment region 2 into the n-type drift path region 1, resulting in adepletion of the n-type drift path region 1. In this case, one end ofthe depletion layer is widened from the a p-n junction ja along a pathlength in the n-type drift path region 1 and the other end thereof iswidened from the p-n junction jb along a path width in the n-type driftpath region 1. That is, the depletion layer is widened from both sidesto accelerate the depletion, and at the same time the p-type compartmentregion 2 is also depleted. Especially in the present invention, asdescribed above, the depletion region can be extended from bothlongitudinal sides of the second conductive type compartment region 2into the adjacent regions 1, 1, respectively. Elongating ends of thedepletion region act effectively on the respective divided drift pathregions 1, 1, so that the total width of the second conductive typecompartment region 2 to be required for forming the depletion layer maybe reduced. Meanwhile, the cross sectional area of the first conductivedivided drift path region 1 may be increased by about the same extent,resulting in a drop in the ON resistance compared with the conventionaldevice. In addition, the trade-off relationship between the ONresistance and the breakdown voltage can be relaxed roughly proportionalto increase the number of the n-type divided drift path regions 1 perunit area (i.e., the number of divided regions per unit area)

For the sake of clarity, the ON resistance R of the vertical MOSFET ofthe present embodiment will be compared with that of the conventionaln-channel MOSFET of FIG. 3 with the ideal breakdown voltage BV=100 V.

In the case of the conventional one, the ideal ON resistance R isapproximately 0.6 (m·ohm·cm²) according to the line A in FIG. 4. In thecase of the present embodiment, on the other hand, the ideal ONresistance R is 1.6 (m·ohm·cm²) when W=10 μm; 0.16 (m·ohm·cm²) when W=1μm; and 0.016 (m·ohm·cm²) when W=0.1 μm if a depth (path length) of eachof the n-type drift path regions 1 and the p-type compartment regions 2as 5 μm and βis to ⅔. Therefore, a noticeable reduction of the ideal ONresistance R can be further obtained. If the width of the p-typecompartment region 2 is slightly larger than that of the n-type driftpath region 1, a further noticeable improvement in the ideal ONresistance R can be obtained. For the mass-production of semiconductordevices, by the way, it is difficult to obtain the width of each region1 or 2 less than 0.5 μm by means of photolithography and ionimplantationat the present time. In the near future, however, further reduction inthe ON resistance of the vertical MOSFET of the present embodiment willbe achieved by lessening the wide of each region 1 or 2 less than 0.5 μmas micromachining technology progresses.

Compared with the lateral semiconductor structure, the verticalsemiconductor structure having the alternating of n-type divided driftregions 1 and p-type divided compartment regions 2 arranged in thevertical direction may be difficult to fabricate. However, it may bepossible to fabricate the vertical semiconductor structure by theprocess including the steps of: forming an-type layer on a drain region29 by means of epitaxial growth; removing the predetermined portions ofthe n-type layer by means of etching to form a plurality of grooves inthe shape of stripes at established spacing; and molding the etchedgrooves by means of p-type epitaxial growth and removing undesiredportions, or by the process including the steps of selectivelyimplanting neutrons or high energy particles having long ranges andperforming a nuclear transformation of the implanted particles toselectively form a deep reverse conductive region.

The invention has been described in detail with respect to variousembodiments. The structure associated with the present invention is notlimited to the drain/drift region of MOSFET described above. It is alsopossible to use a semiconductor region which becomes a depletion regionif the device is in the OFF mode and also becomes a drift region if thedevice is in the ON mode, and furthermore most of the semiconductorelements such as an IGBT, a bipolar transistor, a semiconductor diode, aJFET, a thyristor, a MESFET, and a HEMT. According to the presentinvention, the conductive type can be changed to a reversed conductivetype in case of necessity. In FIGS. 5A to 5C, there are the structureshaving a plurality of divided substructures in parallel, such as alayered structure, a fiber structure, and a honeycomb structure,respectively, but not limited to these shapes. It is also possible touse other shapes.

As described above, the present invention has the features including: aset of first conductive type drift regions provided in a paralleldivided structure, wherein in each of the divided drift regions a driftcurrent flows if it is in the ON mode while each divided drift region isdepleted if it is in the OFF mode; and a second conductive typecompartment region placed in an interface of the side surfaces of theadjacent drift regions to form a p-n junction.

Accordingly, the present invention produces at least the followingeffects.

(1) The depletion region can be widened from both longitudinal sides ofthe second conductive type compartment region into the adjacent regions,respectively. Widening ends of the depletion region act effectively onthe respective divided drift path regions, so that the total width ofthe second conductive type compartment region to be required for formingthe depletion layer may be reduced, while the cross sectional area ofthe first conductive divided drift path region may be increased by aboutthe same extent, resulting in a drop in the ON resistance compared withthe conventional device. Accordingly, the trade-off relationship betweenthe ON resistance and the breakdown voltage can be eased if the numberof the first conductive type divided drift path regions per unit area(i.e., the number of divided regions per unit area) is increased.

(2) The drift region can be fabricated as a stripe structure byalternately arranging the respective first conductive type divided driftpath regions in the shape of stripes and the respective secondconductive type compartment regions in the shape of stripes on a plane.The stripe shaped p-n junction repeated structure on the plane may beformed by performing a photolithograph one time, resulting in a simplemanufacturing process and a low production cost for the semiconductordevice.

(3) The drift region to be provided in the lateral type semiconductordevice may be a superposed parallel structure by alternativelylaminating the respective first conductive type divided drift pathregion in the shape of a flat layer and the respective second conductivetype compartment region in the shape of a flat layer. A thickness ofeach layer can be precisely decreased as much as possible using a metalorganic chemical vapor deposition (MOCVD) or a molecular beam epitaxy(MBE), so that the trade-off relationship between the ON resistance andthe breakdown voltage can be substantially eased.

(4) The most simple drift region of the lateral type semiconductordevice comprises a first conductive type divided drift region formed ona second conductive type semiconductor layer, a well-shaped secondconductive type compartment region formed on the first conductive typedivided drift path region, another first conductive type divided driftpath second region formed on a surface layer of the second conductivetype compartment region and connected to the first conductive typedivided drift path region. The ON resistance of the semiconductor devicecan be reduced by connecting the another first conductive type divideddrift path region with the first conductive type divided drift pathregion in parallel. In this structure, there is no reverse conductivetype layer adjacent to the upper layer of the secondary first conductivetype divided drift path region. Hence, the depletion can be easilyobtained with the decrease in a thickness of the layer.

(5) The fabrication method of the present invention does not require thestep of doping impurities. The second n-type divided drift region isprovided only by the step of the thermal oxidation, contributing toreduced cost and a reduced number of steps for the practical massproduction of semiconductor devices.

(6) The drift region of the vertical type semiconductor device comprisesa plurality of first conductive type divided drift regions and aplurality of second conductive compartment regions, in which each regionis in the shape of layer in the vertical direction. The respective firstconductive type divided drift regions and the respective secondconductive type compartment regions are alternately stacked in parallelin the lateral direction. In the process for fabricating this structure,an etching step can be required for forming a deep groove. In this case,however, it is also possible to substantially ease the trade-offrelationship between the ON resistance and the breakdown voltage of thevertical type semiconductor device.

What is claimed is:
 1. A semiconductor device comprising a drift region,said drift region having a drift current flowing if said drift region isin an ON mode, said drift region being depleted if said drift region isin an OFF mode, said drift current flowing in a lateral direction, saiddrift region being formed as a parallel striped structure comprising aplurality of stripe-shaped first conductivity type drift path regionsand a plurality of stripe-shaped second conductivity type compartmentregions alternately arranged on a plane in parallel and adjacent to oneanother, wherein said drift region is sandwiched by insulation films. 2.A semiconductor device as claimed in claim 1, wherein one of saidinsulation films is thicker than the other and contacts a semiconductorsubstrate.
 3. A semiconductor device comprising a drift region, saiddrift region having a drift current flowing if said drift region is inan ON mode, said drift region being depleted if said drift region is inan OFF mode, said drift current flowing in a lateral direction, saiddrift region being formed on a semiconductor or an insulation film onsaid semiconductor, said drift region being formed as a parallel stripedstructure comprising a plurality of stripe-shaped first conductivitytype drift path regions and a plurality of stripe-shaped secondconductivity type compartment regions alternately arranged on a plane inparallel and adjacent to one another, wherein said second conductivitytype compartment regions are connected to a further region of secondconductivity type.
 4. A semiconductor device comprising a drift region,said drift region having a drift current flowing if said drift region isin an ON mode, said drift region being depleted if said drift region isin an OFF mode, said drift current flowing in a lateral direction, saiddrift region being formed on a semiconductor region, said drift regionbeing formed as a parallel striped structure comprising a plurality ofstripe-shaped first conductivity type drift path regions and a pluralityof stripe-shaped second conductivity type compartment regionsalternately arranged on a plane in parallel and adjacent to one another,wherein an impurity concentration of said semiconductor region, on whichsaid drift region being formed, is lower than impurity concentrations ofsaid first conductivity type drift path regions and impurityconcentrations of said second conductivity type compartment regions.